Photoreceptor is the main component responsible for the formation of images in electrophotographic processes that take place in copiers and laser printers. There are two main types of photoreceptors: those containing inorganic photoconductive materials and those containing organic photoconductive materials. Inorganic photoconductive materials, such as selenium, silicon, and alloies of arsenic and selenium, have been developed and used as photoreceptors in electrophotography. Although appreciable sensitivity can be achieved, the inorganic photoconductive materials suffer from several major drawbacks, such as toxicity and high cost of production. Organic photoconductive materials, on the other hand, offer many attractive features that include relatively low cost, nontoxicity, broad spectral response ranging from visible to near-infrared light, and flexibility of being able to vary their molecular structure and photoconductive properties. A charge-transfer complex of poly(vinyl carbazole) and trinitrofluorenone was the first commercial organic photoreceptor used in copiers. Subsequent research work on organic photoconductive materials has resulted in the development of an aggregated photoconductor based on thiapyrylium salt. Major research efforts have been directed to the enhancement of efficiency of charge supply in organic photoconductive materials.
Stringent requirements are imposed on the photoconductive as well as mechanical properties of electrophotographic photoreceptors. Suitable candidate materials for use as photoreceptors are required to exhibit, not only efficient charge generation and charge transport properties, but also structural integrity and robustness so as to withstand mechanical abrasion during image development cycles. Most commercial copiers and laser printers nowadays have used function-separated photoreceptors to achieve the various requirements. In dual-layer photoreceptors, a charge generation layer and a charge transporting layer are constructed separately and, therefore, versatile choices can be made for photoreceptors to achieve the desired and improved performance. Organic compounds such as azo, bisazo, and perylene pigments are usually chosen as the charge generation material for photoreceptors that are sensitive to visible light. For printers that utilize semiconductor lasers or light-emitting diodes as the light source, photoreceptors are required to be photosensitive to near-infrared beam in the wavelength range of 750 nm to 850 nm. In this category, metal-free phthalocyanines, metallophthalocyanines, and squaraines have been found to exhibit substantial photosensitivity in the near-infrared ranges.
Among the infrared-sensitive organic materials, titanyl phthalocyanines are especially of interest because of their high efficiency of charge generation. The structure of titanyl phthalocyanine is shown in FIG. 1. It was reported that the efficiency of charge generation of titanyl phthalocyanine can reach as high as 94%. It is of technological as well as commercial interests to develop methods that can cost-effectively mass produce the highly infrared-sensitive titanyl phthalocyanine, with consistent and further improved quality, for use as efficient electrophotographic photoreceptors.
The efficiency of charge generation of titanyl phthalocyanine was found to be strongly dependent on its crystal structure. Most of the reported methods for modifying the crystal structure of titanyl phthalocyanines involve an initial acid-pasting treatment step, in which strong protonic acids, such as sulfuric acid and trifluoroacetic acid, are used to dissolve the titanyl phthalocyanine material, and the resultant solution was subsequently precipitated in a nonsolvent such as water, to obtain amorphous powder of titanyl phthalocyanine. Another method for modifying the crystal structure of titanyl phthalocyanines utilizes the ball milling of the dry titanyl phthalocyanine material; this method was shown to have the same effect to obtain the amorphous phase titanyl phthalocyanine.
The essence of the methods reported in the art for the modification of the crystal structure of titanyl phthalocyanine lies in the subsequent solvent treatment of the amorphous titanyl phthalocyanine. Effective organic solvents include halogen-containing hydrocarbons, such as monochlorobenzene, dichlorobenzene, and dichloroethane, etc.; and ethers such as n-butyl ether, tetrahydrofuran and ethylene glycol n-butyl ether, etc.
In U.S. Pat. No. 5,132,197, it was disclosed that highly photoconductive titanyl phthalocyanine can be obtained by treatment with methanol, followed by a wet milling in n-butyl ether or pinene or ethylene glycol n-butyl ether. In U.S. Pat. No. 5,225,551, it was disclosed that the preferred crystal forms of titanyl phthalocyanine can be obtained by precipitation of titanyl phthalocyanine in trifluoroacetic acid/methylene chloride solution with a nonsolvent such as methanol/water with varying volume ratios. Another method that involves wet milling of titanyl phthalocyanine in solvents such as dichloroethane and dichlorobenzene in a wet cake form after precipitation with water was reported in U.S. Pat. Nos. 4,898,799 and 5,114,815. The importance of the presence of water during the modification of crystal forms of titanyl phthalocyanine was further addressed in U.S. Pat. No. 5,298,617. A number of other reports also disclosed that water molecules might have actually become associated with titanyl phthalocyanine after the treatment processes reported in the prior art.
Although highly photoconductive titanyl phthalocyanines have been reported in the art, the mechanism of crystal transformation and the role of different solvent media remained unclear in the above-mentioned prior art disclosures. Furthermore, the possibility of water doping on the titanyl phthalocyanine molecule cannot be neglected. It was also shown in many reports that the electric conductivity of titanyl phthalocyanine and other related phthalocyanines has increased after treatment with water since water, like oxygen, can oxidize the titanyl phthalocyanine material. Therefore, it can be expected that the dark decay may become a serious problem when the water-treated titanyl phthalocyanine is used as the charge generation component of the photoreceptor. The adverse effect of substantial dark decay associated with the titanyl phthalocyanine produced by the prior art processes will cause undesirable foggy images to be produced in the printed copies.